Alkyl glycosides are low-irritant surface-active agents of the sugar-derivative type having excellent biodegradation capacity. In spite of their nonionic nature, they generate stable foams and exert action on other anionic surface active agents as a foam stabilizer. Because of these excellent properties, alkyl glycosides have been widely used as base materials in household detergents.
Alkyl glycosides are synthesized by a process in which a sugar is directly reacted with a higher alcohol in the presence of an acid catalyst (JP-A-59-139397 corresponding to U.S. Pat. No. 4,923,976, European Patent No. 0,132,043) or by a process in which a sugar is reacted first with a lower alcohol such as butanol to obtain a lower alkyl glycoside which is then reacted with a higher alcohol (JP-A-64-71895 corresponding to U.S. Pat. No. 4,990,605, JP-A-64-71896 corresponding to U.S. Pat. No. 4,898,934, JP-A-64-71897 corresponding to U.S. Pat. No. 4,847,368). (The term "JP-A" as used herein means an "unexamined published Japanese patent application)
Since alkyl glycosides have sugar skeletons in their structures, they are useful as surface active agents which satisfy the current trend toward the use of natural materials which are environmentally safe. Opposed to such utility, however, they suffer from the disadvantage that their aqueous solutions are apt to give rise to microbial propagation.
The prevention of putrefaction and fungal growth in surface active agents has been achieved by the addition of antiseptic and fungicidal agents which include, for example, (a) para-hydroxybenzoic acid esters such as methyl para-hydroxybenzoate, ethyl para-hydroxybenzoate and the like; (b) benzoic acids such as benzoic acid, sodium benzoate and the like; and (c) isothiazoles such as 1,2-benzisothiazoline-3-one and the like.
However, para-hydroxybenzoic acid esters have low solubility in water and are incorporated into micelles of surface active agents in emulsions, thus resulting in an insufficient effect against microorganisms in the water layers. As for benzoic acids, they are effective only within the acidic pH range and therefore are not suitable for the antiseptic and fungicidal treatments of alkyl glycosides. What is more important, the amounts of these two types of compounds which can be incorporated into products are limited from the viewpoint of safety.
In addition, isothiazoles are expensive and therefore cannot be used in cosmetics.
Thus, great concern has been directed toward the development of an effective method to protect alkyl glycoside aqueous solutions from putrefaction and fungal growth, and there remains a need for methods and compositions which overcome this problem.